In particular for tools rotating at high speed, like, e.g., cutters or drills, it is known to shrink their shafts into a sleeve section of a tool holder. The sleeve section is heated for this purpose by an induction coil surrounding it, so that the tool shaft can be inserted into the receiver opening of the sleeve section, which expands or enlarges under heat influence. The outer diameter of the tool shaft is slightly larger than the nominal diameter of the receiver opening, so that the tool is held torque proof in the press fit of the tool holder after the cooling of the sleeve section.
An inductive heating device suitable for this purpose is, e.g., the heating device known from DE 101 02 710 A1. This device comprises an induction coil, which can be placed over the sleeve section of the tool holder, and which thus encloses the tool holder at a radial distance in an annular manner, which induction coil is fed by AC power. The magnetic field of the induction coil induces inductive currents in the electrically conductive, mostly also magnetizable material of the tool holder, which induction currents directly heat the sleeve section. The induction coil extends axially at least over the engagement length, by which the tool shaft reaches into the receiver opening and its winding terminates in the portion of the tool side face of the sleeve section axially with the sleeve section. In a radial direction, the inner circumference of the induction coil extends at a distance from the sleeve section in order to be able to use the same induction coil in tool holders with a different outer diameter of the sleeve section.
At its face sides and at its outer circumference, the winding of the induction coil is enveloped by a flux concentration assembly made of a magnetizable (ferromagnetic or ferrimagnetic material), whose high magnetic conductivity compared to air concentrates the magnetic flux substantially to this envelope. The magnetizable material of the flux concentrator assembly is electrically nonconductive in order to prevent that the flux concentrator envelope heats also up inductively. The portion of the flux concentration envelope adjacent to the tool side end of the sleeve section is configured as a substantially annular concentrator body, which directly contacts the tool side face of the sleeve section in axial direction. It is the purpose of such a concentrator body to induct the field into the sleeve section in a controlled manner, in order to let it become effective therein and to prevent at the same time that the tool shaft protruding from the sleeve section is inductively heated by the scattered field portion, which would also cause the tool shaft to expand, which is undesirable. Advantageously, such a concentration body is provided as a shielding collar, approximately as described in DE 101 02 710 A1. The shielding collar captures a substantial portion of the field lines, which have initially exited into the exterior space. Thus, it effectively prevents that the outward protruding tool shaft gets heated up.
Nevertheless, even in such an assembly, a measurable scatter field exists in the exterior of the induction coil or of the entire device. However, this scatter field may not influence the primary function of shrinking in and shrinking out in the major portion of the applications.
Recently, people not closely involved with the matter have become more interested in electric “scatter fields”.